patch 8.0.1339: no test for what 8.0.1335 fixes

Problem:    No test for what 8.0.1335 fixes.
Solution:   Add a test. (Yasuhiro Matsumoto, closes #2373)
diff --git a/src/testdir/test_writefile.vim b/src/testdir/test_writefile.vim
index 9703323b..b8e0001 100644
--- a/src/testdir/test_writefile.vim
+++ b/src/testdir/test_writefile.vim
@@ -100,3 +100,11 @@
   call writefile(['two'], 'Xtest', 'S')
   call delete('Xtest')
 endfunc
+
+func Test_writefile_sync_dev_stdout()
+  if !has('unix')
+    return
+  endif
+  " Just check that this doesn't cause an error.
+  call writefile(['one'], '/dev/stdout')
+endfunc